Alright, he's made it to another sunset.
He ate around 1/4-1/2 cup of straight bird seed throughout the day and drank a good bit of water. The poop was quite the mess since it was two days worth, so I didn't get any pictures of the latest. However, it remained fairly firm throughout the day, though the coconut oil seems to make it watery at times (or perhaps it's something else?). The color is still the same, nothing to report there. I also setup a 'tray' under his cage so its easier to clean regularly and know which is new and which isn't (poop).
He was up and awake most of the day too, with only a couple of light naps (though who is to blame him, stuck inside all day).
Overall I'd say he's at least hanging in there for now, and it's thanks to everyone who has given me advice so far. Thank you all so much! I will post some more updates tomorrow.
EDIT: I haven't started the Corid yet, but will get it to the whole flock tomorrow, as well as some grit and oyster shell stations in the coop. Going to worm the entire flock in the coming days just to be safe as well.